1. Field of the invention
The present invention relates to a printing apparatus and a mechanism for removably supporting a platen roller when a printer head is brought into press-contact with the platen roller to interpose printing paper between the printer head and the platen roller, and more particularly, the invention relates to a thermal printing apparatus (thermal printer) capable of enhancing operability for the loading of the printing paper, head cleaning and the like.
2. Description of the Related Art
A thermal printer is constituted to form an image on a thermal printing paper by using a thermal head.
FIG. 8 shows an example of the structure of a conventional thermal printer and a printing paper inserting path. A platen roller 2 is rotatably supported to a body frame 1 through a bearing (not shown), and a thermal head 5 is rotated and biased to come in press-contact with the platen roller 2 by the biasing force of a pressure spring 8. The thermal head 5 can be separated from the platen roller 2 to form a clearance between the thermal head 5 and the platen roller 2 at the time of the loading of the printing paper, maintenance such as head cleaning, and the like. The printing paper is inserted in the clearance and is thus loaded.
FIG. 9 is a side view illustrating a mechanism for removably supporting the platen roller which has a problem to be solved by the present invention. FIG. 9 shows an example of the structure of a mechanism for pushing a bearing 3 of the platen roller 2 against the body frame 1 by the pressing force of the thermal head 5, thereby engaging the platen roller 2. The platen roller 2 is provided across the body frame 1 of the printer rotatably and removably. A line type thermal head 5 fixed to the front face of a head support member 6 is biased to come in press-contact with the platen roller 2. The thermal head 5 is arranged almost vertically in the body frame 1. A portion 1c of the body frame 1 for engaging the platen roller 2 has the shape of a special slit in order to attach or remove the platen roller 2.
Referring to the structure in which the platen roller 2 cannot be attached to or removed from the body frame 1 as shown in FIG. 8, conventionally, there have been a method of bringing the tip of a printing paper to a contact point at which the thermal head 5 is in press-contact with the platen roller 2 and rotating the platen roller 2 to draw the printing paper for loading and a method of separating the thermal head 5 from the platen roller 2 and inserting the printing paper in the clearance formed therebetween. However, these methods require a user to know well how to handle the thermal printer. In particular, operability related to the insertion of the printing paper has had a big problem in that it is hard to grope for a printing paper insertion port in a dark environment, for example, at night. Moreover, in the case where maintenance such as head cleaning is to be carried out, an applicator or the like can be put in the clearance between the thermal head 5 and the platen roller 2. However, there has been a possibility that visual confirmation might be difficult and cleaning might be carried out insufficiently because of a small opening.
In order to solve these problems, there has been devised the structure in which the platen roller 2 can be completely attached to or removed from the body frame 1 as shown in FIG. 9. When the printing paper is to be loaded, the platen roller 2 is lifted up from the body frame 1 and is separated therefrom, the printing paper is dropped into the printer body and the platen roller 2 is then attached to the body frame 1 again as shown in FIG. 9. Thus, printing paper is loaded between the thermal head 5 and the platen roller 2. With such a structure, however, the platen roller 2 is engaged with the frame 1 by the biasing force of the thermal head 5 pressing the platen roller 2. The structure has low positioning precision for the platen roller 2. Moreover, a larger operating force is required for the attachment and removal of the platen roller 2. In the case where the platen roller 2 is not pushed into a predetermined position during the attachment of the platen roller 2, a friction is generated between the body flame 1 and the bearing 3 of the platen roller 2 by the pressing force of the thermal head 5 so that the platen roller 2 might be fixed into a position that is not the position specified. When the support position of the platen roller 2 is shifted, a driving mechanism is not operated normally. In some cases, a print failure is caused.